My first lesson learned -- after re-starting the first seven rows five times -- is SSK. This is a left-side decrease, so a pattern row will say something like, knit two, knit two together (right-side decrease), knit one, SSK, knit two, thus forming a triangular shape around the center stitch.
I looked up SSK and learned it stood for "slip, slip, knit." So, I go my happy little way and get to the first set of SSK stitches and suddenly I'm short a stitch. I re-did the math five times. The pattern wasn't wrong. Somehow, some way, my careful focus and stitch count got off.
Four re-dos later and I was about to need a Tylenol. How could 11 stitches drive someone so crazy?!
I consulted YouTube to figure out where I went wrong. Suddenly it became clear. I was doing SSK as if it were three stitches: slip one, slip one, knit one. SSK only uses two stitches! You slip two stitches, stick the left-hand needle back through them and knit them together. That's where the "knit" part came in!
If you're more of a visual person, here's the fabulous video that told me how to SSK the right way, courtesy of PurlBee: